Biaxially oriented polypropylene (BOPP) films used for packaging, decorative, and label applications often perform multiple functions. For example, they may perform in a lamination to provide printability, transparent or matte appearance, or slip properties. Further, they sometimes may provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
For matte BOPP films, it is often desirable for the matte layer to have a certain low gloss level and a hazy appearance. The level of matte appearance is an aesthetic consideration and can be achieved by mixing two incompatible polymers. In most cases, this can be a blend of ethylene homopolymer (either low density polyethylene, medium density polyethylene, or high density polyethylene) with a propylene homopolymer or copolymers (e.g. propylene-ethylene copolymer, propylene-ethylene-butene copolymers, etc.). The degree of matte appearance can be controlled by the amount of polyethylene blended with the propylene homopolymer or copolymer.
EP Patent 0538747 B1 describes the formulation of a matte resin blend using HDPE and a copolymer of ethylene-propylene, ethylene-butylene, propylene-butylene, or ethylene-propylene-butylene in a weight ratio of 20-80% HDPE. A sealable matte resin formulation is produced by combining a specific blend of HDPE and ethylene, butene, or propylene co- or terpolymers. A large amount of co- or terpolymer is required to achieve heat sealability (40-55 wt %). However, no or little sealability can occur if the matte resin is formed from HDPE (or LDPE, MDPE) and a propylene homopolymer. In addition, if the amount of propylene co- or terpolymer is too high in order to achieve sealability, matte appearance is diminished.
EP Patent 0622186 B1 describes the formulation of a sealable and shrinkable matte BOPP film using a matte resin blend of HDPE and blends of ethylene, butene, or propylene co- or terpolymers in a weight ratio of 20-80% HDPE. The film exhibits specific machine and transverse direction shrinkage values which are relatively high for a BOPP film at specified temperatures and thus, may be unsuitable for non- or low-shrinkage thermally stable BOPP applications. However, no or little sealability can occur if the matte resin is formed from a HDPE (or LDPE, MDPE) and a propylene homopolymer. Moreover, if the amount of propylene co- or terpolymer is too high in order to achieve sealability, matte appearance is diminished.
U.S. Pat. No. 4,522,887 describes a matte film formulation using a matte resin blend of ethylene-propylene block copolymer of at least 3 melting peaks within the range of 120° C. to 165° C. and including 10-50 wt % ethylene. HDPE or other ethylene homopolmers blended with propylene copolymer or homopolymer are not used to impart matte appearance. The patent is silent with respect to heat sealable properties of the invention, but generally speaking, block copolymers will have a higher seal initiation temperature and thus, a narrower seal range, than respective random copolymers.
EP Patent 1268202 B1 describes a matte film formulation using a matte resin blend formulation of 30-80 wt % high molecular weight HDPE and an incompatible polyolefin such as ethylene-propylene copolymer, ethylene-propylene-butene terpolymer, or propylene homopolymer. Some of the examples in this patent exhibited heat sealability but only at 116° C. or higher, which limits the utility of such heat sealability in high-speed sealing applications.
EP Patent 0538746 B1 describes a matte film formulation using a matte resin formulation of 20-80 wt % HDPE and propylene homopolymer. The film has a high haze of >40% and is not heat sealable.
EP Patent 0622187 B1 describes a matte film formulation using a matte resin formulation of propylene homopolymer or ethylene, propylene, or butene copolymers with HDPE further blended with propylene homopolymer or ethylene, propylene, or butene copolymers. The film is also highly shrinkable at specified temperatures. However, no or little sealability may occur if the matte resin is formed from HDPE (or LDPE, MDPE) and a propylene homopolymer. In addition, if the amount of propylene co- or terpolymer is too high in order to achieve sealability, matte appearance is diminished.